1. Field of the Invention
The present invention relates to distributed control systems and more particularly to a control technique wherein control information is embedded in the information that is communicated between elements of a system.
2. Description of the Prior Art
The trend in modern telephony equipment designs has been to incorporate microprocessors as system control elements. In most cases, such equipment must support numerous replicated elements which provide standard telephony services or interface standard telephony signals such as DS1. Such elements are often designed to minimize cost due to the impact of replication and system cost. As a result, these elements are often designed as non-intelligent elements having simple, register-oriented control (ROC) interfaces; and which must be controlled by a common control element whose cost can then be effectively distributed across the replicated elements.
A distributed control system architecture may use multiple, distributed microprocessors (control elements) which must communicate with one another and with ROC interfaces in order to co-ordinate system operations. These intelligent control elements often communicate with each other using higher-level, message-oriented control (MOC). A challenging aspect of such architectures is to provide a mechanism for controlling the numerous distributed elements, both intelligence and non-intelligent, in a flexible, but cost-effective, manner.
Non-intelligent elements which employ ROC must receive control information from an intelligent control element. The non-intelligent element normally responds to control information immediately upon reception thereof. Non-intelligent elements also produce information which must be read and acted upon by the control element. This information may fall into one of two categories: urgent information which must be acted upon quickly, and other information which may be referenced on an as-needed basis or periodically scanned at an appropriate rate.
Intelligent elements which communicate via MOC typically send messages in order to report significant system events. Since the information carried in messages may be urgent in nature, the receiving control element must be notified upon arrival of a message in order to promptly initiate message interpretation. Also, since the number and frequency of messages handled may be large, it is often important to minimize the burden of message transmission and reception on the control elements.
A common method of implementing MOC is by using commercially available Data Link Control devices. These devices are both costly and bulky, and separate devices, or a separate channel of a multi-channeled device, must be dedicated to each full duplex MOC channel. Each such device requires interface circuitry to synchronize its transmission and reception. Also, a Direct Memory Access (DMA) controller for each channel is usually needed to free the control element from handling each individual message character. While this technique may be acceptable for a small number of interconnected intelligent system elements, the cost and physical space required may become prohibitive in systems having a large number of elements.
For non-intelligent ROC elements, there are a myriad of techniques for implementing ROC, each one of which is typically customized and optimized to the type of element being controlled. In most cases where both ROC and MOC are present, the ROC technique is different and more primitive than the MOC technique. It is often required that the control element explicitly request information from the controlled element when needed. If information is needed frequently in a system with many replicated elements, the repetitious request/response dialogue may represent a significant overhead to the control element.